A radar device radiates a radio wave from a measuring point into a space, receives a signal of a reflected wave that is reflected from a target, and measures the distance between the measuring point and the target, the direction, and the like. Recently, particularly, a radar device which can detect not only an automobile, but also a pedestrian or the like as a target by a high-resolution measurement using a short-wavelength radio wave such as a microwave or a millimeter wave has been developed.
Moreover, a radar device sometimes receives a signal in which a reflected wave from a target at a short distance and that from a target at a long distance are mixed with each other. In the case where a range side lobe is produced by a signal of a reflected wave from a target at a short distance, particularly, the range side lobe is sometimes mixed with a main lobe of a signal of a reflected wave from a target at a long distance. In this case, the accuracy of detection in which the radar device detects the target at a long distance may be impaired.
In the case where an automobile and a pedestrian are at the same distance from a measuring point, moreover, a radar device sometimes receives a signal in which signals of reflected wave from the automobile and pedestrian having different radar cross sections (RCS) are mixed with each other. Usually, the radar cross section of a pedestrian is smaller than that of an automobile.
Therefore, a radar device is requested to, even in the case where an automobile and a pedestrian are at the same distance from a measuring point, properly receive not only a reflected wave from the automobile, but also that from the pedestrian. As described above, a radar device is requested to have a reception dynamic range which is so wide that signals of reflected waves reflected from targets that cause various reception levels depending on the distance and kind of a target can be received.
Therefore, a radar device which must perform a high-resolution measurement on a plurality of targets is requested to transmit a pulse wave or pulse modulated wave having characteristics in which the autocorrelation characteristics are in the low range side lobe level (hereinafter, referred to as “low range side lobe characteristics”).
As an example of the above-described radar device, known is a radar device which has a plurality of radar units, and in which the radar units independently measure respective predetermined measurement areas to measure a wide-angle range as a whole, thereby detecting a target. In the following description, each of radar devices which, in detection of a target, measure respectively different predetermined measurement areas is referred to as “radar unit.” Measurement areas of radar units are different from each other, but, in the case where the measurement areas are in proximity to each other, may sometimes partly overlap each other.
In a conventional radar device, in the case where measurement areas of radar units are in proximity to each other, interference occurs between transmission signals transmitted from the radar units. When interference occurs between transmission signals in a conventional radar device, there is a problem in that accuracy of position estimation of a target is impaired.
For example, Patent Document 1 is known which discloses an apparatus that reduces interference between transmission signals of radar units in order to solve the problem. An outline of the conventional radar device disclosed in Patent Document 1 will be described with reference FIG. 25. FIG. 25 is a view showing a timing chart illustrating the operation of the conventional radar device.
The radar device of Patent Document 1 has two radar devices or A radar device and B radar device. The A radar device has a synchronizing section which controls the timing of A pulse signal that is to be emitted from the A radar device, and an I/F section which receives B synchronous trigger signal synchronized with B pulse signal that is emitted from the B radar device. The A radar device receives the B synchronous trigger signal through the I/F section. The A radar device controls the timing of emission of the A pulse signal to be emitted by the A radar device, based on the received B synchronous trigger signal.
As a result of this control, as shown in FIG. 25, the arrival time period of the interference wave of the A radar device, i.e., an interference signal which is received as the interference wave of the A radar device by the B radar device is fixed to the outside (Tx) of the time interval of the reception effective period of the B radar device. Therefore, the interference wave from the A radar device which is received by the B radar device does not affect the measurement of the B radar device.
Moreover, the arrival time period of the interference wave of the B radar device, i.e., an interference signal which is received as the interference wave of the B radar device by the A radar device is within the reception effective period of the A radar device. However, it is assumed that the A radar device performs a limited interference suppressing or gating process on the interference signal, thereby enabling the interference signal to be effectively removed. In FIG. 25, the parameter Tm indicates the reception effective period, the parameter Tx indicates the time interval between reception effective periods, and the parameter Td indicates the time period elapsed before arrival of an interference wave from another radar device.